The present invention concerns a process for the manufacture of organic isocyanates from nitro compounds and more specifically a process for the preparation in the liquid phase of aromatic isocyanates by reaction between aromatic nitro derivatives and carbon monoxide in the presence of heterogeneous catalysts constituted of metallic particles or metallic oxides, deposited on supports, originating from heteropolymetallic complexes.
Aromatic isocyanates are organic intermediates of great interest. Two among them have been particularly well developed; namely, toluene diisocyanate and diphenylmethane-4,4'-diiosocyanate which are used in the synthesis of polyurethanes. Certain substituted aromatic isocyanates are likewise used in the syntheses of herbicides. Industrial processes for the preparation of these products all involve the phosgenation reaction of an amine originating from the catalytic hydrogenation of a nitro derivative. The drawbacks of these processes are several. More particularly, they necessitate the synthesis and manipulation of phosgene, a very dangerous product, and they produce important quantities of hydrochloric acid as a by-product requiring the installation and costly upkeep of a special shop for the electrolysis of this acid in order to recycle the chlorine and hydrogen.
The advantage which a process avoiding the use of phosgene presents is obvious. Several patents describe catalytic compositions making it possible to prepare the isocyanates at elevated temperature and pressure by reaction of an organic nitro compound with carbon monoxide. The favorable role of noble metals for this reaction is well known from these publications. Thus, French Pat. No. 1,600,529 describes the use of a precious metal halide as a catalyst, in the presence of a basic amine of aromatic character. German Pat. No. 1,910,303 describes catalysts formed of halides or oxides of Ru, Rh, Pd, Os, Ir, Pt, and of a heteroatomic sulfur-containing compound, possibly in the presence of an oxide of Cr, Mo, Nb, W, V. French Pat. No. 1,567,321 describes the use of a catalytic system formed by a precious metal halide and an organic phosphorous compound; for instance a triaryl phosphine or a phosphite. French Pat. No. 2,155,242 describes catalytic systems consisting of one or several palladium and/or rhodium halides, one or several basic heteroatomic nitrogen compounds, and a co-catalyst formed by one or several iron borates. In French Pat. No. 2,120,110, besides a palladium halide and heterocyclic nitrogen-containing bases, the catalytic formulation includes a co-catalyst consisting of one or several iron and/or manganese molybdates.
All of these systems based on noble metals make it possible to produce isocyanates from nitro compounds with variable selectivities and productivities. However, the difficult and costly recovery of the isocyanates prevents their industrial use.
Certain attempts have been made to deposit the active phases on supports. It is, thus, that in French Pat. Nos. 1,600,529 and 1,558,896, as well as in U.S. Pat. No. 3,728,370, it has been envisioned, without practical illustration, to deposit the catalyst on supports of the following types: alumina, silica, carbon, barium sulfate, calcium carbonate, asbestos, bentonite, diatomaceous earth, fuller's earth. British Pat. No. 1,257,932 describes the use of palladium, palladium chloride, or rhodium chloride on alumina, silica and silicon carbide for carbonylation, in the vapor phase, of nitro derivatives. A publication of W. B. HARDY and R. P. BENNETT in Tetrahedron Letters No. 1, p. 961 (1967) gives the results obtained with a carbonylation of nitrobenzene yielding phenylisocyanate.
Supported catalysts, whose active phase consists of both palladium and molybdenum, are known. Thus, French Pat. No. 2,452,318 describes the carbonylation of aromatic nitro compounds with the help of catalysts consisting of palladium and molybdenum oxides deposited by successive impregnation of ammonium molybdate and palladium chloride on an alumina, silica, or silica/alumina support.